Due to its characteristics of being easily applicable to various products and electrical characteristics such as a high energy density, a secondary battery is not only commonly applied to portable equipment, but universally applied to an electric vehicle (EV) or a hybrid electric vehicle (HEV) propelled by an electric driving source. This secondary battery is gaining attention for its primary advantages of remarkably reducing the use of fossil fuels and not generating by-products from the use of energy, making it a new eco-friendly and energy efficient source of energy.
Recently, with the growing interests in smart grid, a high capacity energy storage system that stores excess energy is required to build an intelligent electrical grid. To build a high capacity energy storage system, a plurality of secondary batteries are connected in series or parallel for required output and charging capacity designs.
As the output and charging capacity required for an energy storage system becomes higher, a number of secondary batteries increases. Also, as the output and charging capacity required for an energy storage system becomes higher, the size of secondary batteries also increases. In this instance, a secondary battery generates heat during charging and discharging, and in the case of a high capacity energy storage system, even a small amount of heat generation may have great influence on an entire energy storage system.
Accordingly, studies are being actively made to minimize the influence of heat generated from secondary batteries constituting an energy storage system.